Recently, a liquid crystal display device used in a mobile device has been required to less consume power as the mobile device has been required to operate for an extended period of time. Here, for example, a mobile device such as a mobile phone is not always in a busy state but is in a waiting state for most of the time. Further, an image and a format displayed in a busy state are usually different from those displayed in a waiting state.
For example, in a waiting state, a liquid crystal display device only needs to be able to display a menu screen, time, and the like and therefore may occasionally have low fineness and a small number of display colors. Rather, it is important for a liquid crystal display device to less consume power so as to operate for an extended period of time. Conversely, in a busy state, a liquid crystal display device usually displays a large quantity of sentences, figures, and images such as pictures and therefore is required to perform high-definition display. At this time, other parts (e.g., a communication module, an input interface section, and an operation processing section) of a mobile device consume a large amount of electric power, so that a display module less consumes power. Therefore, a mobile device is more strongly required to less consume power in a waiting state than in a busy state.
Accordingly, for example, in an attempt to reduce power consumption in a waiting state, Japanese Laid-Open Publication 248468/2003 (Tokukai 2003-248468; published on Sep. 5, 2003) discloses an image display device 100. In the image display device 100, as shown in FIG. 15, a display screen 101 is divided for display, i.e., partial display. In the partial display mode, the display screen is divided into three areas P1, P2, and P3. For example, the areas P1 and P3 serve as nondisplay portions each of which displays nothing but a white background, and the area P2 displays a static image such as time and wallpaper. Therefore, in a waiting state, the area P2 serves as a display portion, and the areas P1 and P3 serve as nondisplay portions. And, in a waiting state, the area P2 and the areas P1 and P3 are driven for display at different refresh rates (rewrite rates). The areas P1 and P3 are driven for display at a lower refresh rate for intermittent writing than the area P2.
This causes the image display device 100 in a busy state to perform high-definition display of a large quantity of sentences, figures, and images such as pictures in a multi-gradation manner and causes the areas P1 and P3 in a waiting state to perform display by more intermittent writing than the area P2 in a waiting state, thereby reducing power consumption.
A driving method of the image display device 100 will be described more in detail based on a timing chart. Note that, a timing chart in a case where partial display is not performed will be described first.
First, as shown in FIG. 16, in a full-screen display mode in which partial display is not performed, a gate start pulse GSP becomes high in voltage for every predetermined number of gate clock signals GCK. That is, the gate start pulse GSP becomes high in voltage in every single vertical scanning period (1V). At this time, in a data signal line driving circuit, a source start pulse SSP becomes high in voltage for every predetermined number of source clock signals SCK, so that a data signal DAT is applied to a pixel after preliminary charging with a pre-charge control signal PCTL. Therefore, in this driving method, the gate clock signals GCK and the source clock signals SCK continually operate, and a refresh rate of a display screen 201 is constant. Further, display is performed in every single vertical scanning period. This undesirably incurs an increase in power consumption.
Conversely, as shown in FIG. 17, in a driving mode in which partial display is performed, the areas P1 and P3 serve as nondisplay portions each of which displays nothing but a white background (white data). Moreover, a refresh rate of the white data can be lowered without raising any display problem. This causes the refresh rate to be lower than that of image data for display in the area P2.
Further, the area P2 performs display once in every three vertical scanning periods (3V). That is, the gate clock signals GCK and the gate start pulse GSP, as well as the source clock signals SCK and the source start pulse SSP, are activated in a first vertical scanning period, and the gate clock signals GCK and the gate start pulse GSP, as well as the source clock signals SCK and the source start pulse SSP, are stopped in a second scanning period and a third scanning period so as to stop circuit operation. A liquid crystal is prone to retain display even when thus driven, so that a static image keeps being displayed.
Furthermore, the white data for nondisplay is displayed in every six scanning periods, and a driving circuit thereof is stopped in a fourth scanning period, thereby further reducing power consumption.
Thus, in the display device of the laid-open publication discloses various techniques for reducing power consumption.
However, as shown in FIG. 17, in the conventional driving method of a conventional liquid crystal display device, the white data for nondisplay on a background in the areas P1 and P3, in a waiting state, is displayed at a low refresh rate but is written by using multi-gradation display data.
Here, when the multi-gradation display data is used, a data signal line driving circuit needs to be driven. The data signal line driving circuit has a shift register, a latch circuit, and a level shifter. The level shifter raises such a problem that an invalid current constantly flows regardless of operations.
Therefore, the arrangement raises such a problem that power is consumed unless the data signal line driving circuit is stopped.